Mishnah Mikvaos 6:10: Regarding a drain [at the bottom of a mikveh] in a bath, when it is in the middle, it invalidates [the mikveh]; when it is on the side, it does not invalidate, since it is like a mikveh adjacent to a mikveh, according to Rabbi Meir. And the Sages say: if the tub can contain a revi'it [a specific measure of volume, a quarter of a log] before they [the waters] reach the drain, it is valid; and if not, it is invalid. Rabbi Elazar in the name of Rabbi Tzadok says: if the drain can contain any amount, it invalidates.

הָאָבִיק שֶׁבַּמֶּרְחָץ, בִּזְמַן שֶׁהוּא בָאֶמְצַע, פּוֹסֵל. מִן הַצַּד, אֵינוֹ פוֹסֵל, מִפְּנֵי שֶׁהוּא כְמִקְוֶה סָמוּךְ לְמִקְוֶה, דִּבְרֵי רַבִּי מֵאִיר. וַחֲכָמִים אוֹמְרִים, אִם מְקַבֶּלֶת הָאַמְבָּטִי רְבִיעִית עַד שֶׁלֹּא יַגִּיעוּ לָאָבִיק, כָּשֵׁר. וְאִם לָאו, פָּסוּל. רַבִּי אֶלְעָזָר בְּרַבִּי צָדוֹק אוֹמֵר, אִם מְקַבֵּל הָאָבִיק כָּל שֶׁהוּא, פָּסוּל:

Mishnah Mikvaos 6:11: Regarding the purifying pools in a bathhouse [used for rinsing off in cold water after a warm bath], where a lower one is filled with drawn waters and an upper one is filled with valid [waters], if there are three log opposite the opening [connecting the two pools], it [the upper pool] is invalid [due to the three log, if it contains fewer than forty se'ah]. How far must the hole span in order for it to contain three log? One three-hundred-twentieth of the pool [i.e. since three is one three-hundred-twentieth of 960, which is the number of log in forty se'ah], according to Rabbi Yose. Rabbi Elazar says: even if the lower one is full of valid [water] and the upper one is full of drawn water, and there are three log opposite the opening, it is valid, for they [the Sages] only said "three log that fell" [into a incomplete mikveh invalidate it; and such a case is not considered falling in].

הַמַּטְהֶרֶת שֶׁבַּמֶּרְחָץ, הַתַּחְתּוֹנָה מְלֵאָה שְׁאוּבִין וְהָעֶלְיוֹנָה מְלֵאָה כְשֵׁרִין, אִם יֵשׁ כְּנֶגֶד הַנֶּקֶב שְׁלשָׁה לֻגִּין, פָּסוּל. כַּמָּה יְהֵא בַנֶּקֶב וְיִהְיֶה בוֹ שְׁלשָׁה לֻגִּין. אַחַד מִשְּׁלשׁ מֵאוֹת וְעֶשְׂרִים לַבְּרֵכָה, דִּבְרֵי רַבִּי יוֹסֵי. וְרַבִּי אֶלְעָזָר אוֹמֵר, אֲפִלּוּ הַתַּחְתּוֹנָה מְלֵאָה כְשֵׁרִים וְהָעֶלְיוֹנָה מְלֵאָה שְׁאוּבִין וְיֵשׁ בְּצַד הַנֶּקֶב שְׁלשָׁה לֻגִּין, כָּשֵׁר, שֶׁלֹּא אָמְרוּ אֶלָּא שְׁלשָׁה לֻגִּין שֶׁנָּפְלוּ: